This application is a 371 of PCT/G02452 filed Nov. 8, 1994.
1. Field of the Invention
This invention relates to the removal of radioactive contamination and, more particularly, to the removal of embedded radioactive contamination from metallic surfaces using laser beams.
2. Discussion of Prior Art
During the operation of nuclear processing plants it is inevitable that surfaces will become contaminated with radioactive substances. Consequently, during the decommissioning of these plants it is necessary to decontaminate the contaminated surfaces in a safe manner. Often the contaminated surfaces comprise stainless steels or mild steels and typical contaminants include UO2, PuO2, Co-60, Sr-90, Cs-134 and Cs-137. The contaminants may be in the form of fine particles or solutions which can penetrate into steel substrates for a distance of about 4 mm. In such situations well known decontamination techniques such as chemical washing, fluid shear blowing or paste/stripping are not effective for the removal of embedded contamination.
One current approach for the reduction of contamination is to maintain a negative pressure within a nuclear containment such that radioactive contamination is confined within specific zones. However, such a scheme has a disadvantage in that running costs are high.
EP0091646 describes a technique for laser (ns pulse) ablation/vaporisation of thin (less than 40 microns) metal oxide films from metal surfaces. The ablation technique is achieved by applying a high energy laser pulse (exceeding 1 GW) to directly break molecular bonds without going through thermal stages. The typical depth of the removed layer is of the order of microns. The laser vaporisation removal is not efficient for metallic surfaces since much heat can be lost through conduction. Again the depth of the removed layer is in the micron range.
Another known technique, described in JP 63024139, uses oft axis gas injection into the laser melt pool for the removal of laser-generated molten materials. This technique can achieve the removal of surface layers of the order of millimetres. However, the alignment of the gas jet relative to the melt pool is critical and when there are object standoff changes the correct alignment is often difficult to achieve. Another disadvantage is that this technique is suited to processing in one direction only.
According to the present invention there is provided a method for the removal of embedded contamination from a metallic surface, the method comprising directing a laser beam on to the contaminated surface, the laser beam having sufficient power density to cause direct ejection of laser-generated melt pool liquid from the metallic surface thereby removing a metallic surface layer containing the embedded contamination.
Preferably, the power density is greater than 6 MW/cm2.
Preferably, the laser beam comprises pulsed energy, eg having a pulse length of at least 1 ms and a pulse energy of 5 J.
The method makes use of laser-generated vapour pressure and optical pressure to achieve the direct ejection of laser molten liquid, and the laser generated vapour recoil pressure is typically between 5 to 100 bar. The molten liquid can be ejected at least 0.1 metre and as far as 2.5 metres from the melt pool.
Conveniently, the metallic surface may comprise stainless steel or mild steel.
Advantageously, the ejection of the laser-generated melt pool liquid is achieved without the use of an additional gas jet blown into the melt pool.
The method can remove a contaminated surface layer to a depth of up to 5 mm.
Desirably, means may be provided for the collection of laser ejected material in order to prevent recontamination of the metallic surface or contamination of previously uncontaminated surfaces and the collection means may comprise an air/water spray and an extraction system.
The laser producing the laser beam may be a gas or a solid state type laser.
The inventors have recognised that since the majority (more than 90%) of embedded contamination is within 1 mm of the surface of contaminated steel, the removal of this surface layer allows the level of contamination to be greatly reduced. The present invention is, therefore particularly advantageous in the safe removal and collection of such embedded contamination.
The present invention is particularly suited to the removal of contamination along a linear path such as that defined by joints, cracks, edges, corners, gaps or the like from which the contamination cannot be washed out or removed by conventional means during the decontamination of metallic nuclear installations.
The present invention may also be used for the removal of contamination from the interior surfaces of metallic pipes or tubes.
The meltpool as produced by the method according to the present invention is strongly radiation-emitting and we have found that the radiation emitted can be detected, digitised and analysed in the method described in a copending International Patent Application of even date by the present applicants claiming priority from GB 9323054.8 the contents of which are incorporated herein by reference. The image produced thereby gives information about the surface orientation, local geometry and standoff distance relative to the heat or laser source producing the meltpool.